Final answer:
Photosynthesis is an endothermic reaction that requires an input of energy. Sunlight is the energy source that drives photosynthesis.
Step-by-step explanation:
Photosynthesis is an endothermic reaction. During photosynthesis, carbon dioxide (CO2) is reduced using electrons from water (H2O), which requires an input of energy.
The energy source that drives photosynthesis is sunlight. Specifically, visible light is used to fuel the reduction of CO2 and the synthesis of carbohydrates.